Radar devices are each mounted mainly to the front portion of a vehicle between a radiator grille and a radiator for monitoring, for example, the forward view of the vehicle. A grill cover is commonly mounted on a grill. Such a grill cover is commonly made of a specific material that has low reflectivity to radio waves, i.e. radar waves, transmitted from a radar device.
Radar devices have been recently needed for monitoring a closer region thereto with a wider field of view. In order to meet these requirements, a radar device is expected to be mounted to the inner side of a bumper cover, the inner side of a front windshield, or the inner side of a resin body of a vehicle.
Unfortunately, these bumper covers, front windshields, and resin bodies commonly have low radar wave transmittance. A bumper cover or a resin body made of materials with high transmittance of radar waves might reduce the aesthetics of the vehicle. A front windshield has difficulty in having high transmittance of radar waves while maintaining its high shatterproof characteristics.
A bumper cover, a front windshield, or a resin body, to which a radar device is mounted, with low transmittance of radar waves may reduce the energy of radar waves that have reached targets, resulting in deterioration of the target-detection capability of the radar device.
A bumper cover or a front windshield, to which a radar device is mounted, with low transmittance of radar waves may also cause radar waves reflected by the bumper cover or the front windshield to be received by a receiving antenna of the radar device. This may reduce the detection reliability of the radar device in detecting targets. In addition, the electrical power of radar waves reflected by a bumper cover or a front windshield is likely be higher than the electrical power of echoes generated based on reflection of transmitted radar waves by a target in front of the vehicle. This may make it difficult for a receiving circuit of the radar device to detect the echoes.
If a radome covers a transmitting antenna of a radar device, the radome may reflect radar waves transmitted from the transmitting antenna.
Accordingly, users have high expectations for the structure of mounting a radar device to a vehicle, which enables efficient reduction of reflection of transmitted radar waves by at least one of vehicle's members, such as a bumper cover, and a radome of the radar device. In particular, it is preferable that the structure of mounting a radar device to a vehicle enables efficient reduction of reflection of transmitted radar waves by at least one of the vehicle's members, such as a bumper cover. This is because the amount of reflection from vehicle's members including a bumper cover is relatively larger than the amount of reflection from the radome.
On the other hand, a non-patent document, “Broadband matching of high-permittivity coatings with frequency selective surfaces”, written by Frerk Fitzek, Ralph H. Rasshofer, Erwin M. Biebl, Proceedings of the 6th German Microwave Conference, Mar. 14-16, 2011, discloses the following technology. Specifically, the technology discloses a dielectric plate-like bumper cover with one major side, on which a frequency-selective surface (FSS) substrate is formed, to reduce reflection of radar waves by the bumper cover. The following refers to the disclosed bumper cover as an FSS bumper cover.